2. Technical Field of the Invention
The invention relates generally to image processing; and, more particularly, it relates to means by which higher quality images may be generated in accordance with such image processing.
3. Description of Related Art
Devices that perform display and processing of digital images exist in a variety of contexts (e.g., computers including desktop, laptop, and handheld computing devices; televisions; media content players including DVD players and set top boxes (STBs); etc.). When processing digital images to perform any up-sampling and/or down-sampling, sometimes the visual quality of the generated/updated digital image is compromised. For example, certain image processing schemes that are directed to preserving high frequency content therein can introduce ringing (sometimes alternatively referred to as rippling or other equivalent terms) near the high frequency content boundaries. This ringing can greatly reduce the quality of the image seen by a user.
Whether the image is a still image being viewed by a user (e.g., on some digital image capable viewing device) or one image of a sequence of images (e.g., such as in digital video stream being viewed on a television (TV)), this degradation of digital images can reduce a user's experience or enjoyment of viewing such digital images.
When scaling an image, in order to preserve high frequencies (sharpness) and prevent aliasing, it is common to use a sharp roll-off multi-tap finite impulse response (FIR) filter. This type of filter will result in a scaled image with accurate high-frequency detail, but will often yield ringing (oscillations) around sharp edges.
In FIG. 2A and FIG. 2B, two input pictures have been scaled up horizontally by a factor of two, using a sharp roll-off multi-tap (oscillating) FIR filter. In FIG. 2C, the picture is the result of an up-scale by 3.
FIG. 2A illustrates an embodiment 200a of a resulting digital image, that includes preserved high frequency content, as generated using only a sharp roll-off, multi-tap finite impulse response (FIR) filter. In FIG. 2A, the high frequency patterns are preserved.
FIG. 2B illustrates an embodiment 200b of a resulting digital image, that includes undesirable ringing/rippling in transition regions/vertical edges, as generated using only a sharp roll-off, multi-tap FIR filter. In FIG. 2B, very undesirable and unwanted ringing or ripples can be seen alongside the vertical edges. While these distortions are expected because of this type of sharp roll-off, multi-tap FIR filter, they are certainly not desirable in the scaled image.
FIG. 2C illustrates an embodiment 200c of a resulting digital image, that includes undesirable ringing/rippling along edges, as generated using only a sharp roll-off, multi-tap FIR filter. In FIG. 2C, the edges are fairly sharp/crisp and clean, but again ringing can be seen along the edges.
Alternative means of performing image processing include using a non-oscillating FIR filter in the scaler (e.g., a linear FIR filter). However, while the use of such a different type of filter will effectively eliminate or reduce rippling and/or ringing, it will either cause the loss of high-frequency details and/or introduce aliasing. In other words, this solution of performing this type of filtering can incur a greater reduction in image quality than the introduction of undesirable and unwanted ringing or ripples as described above.
In FIG. 3A and FIG. 3B, the same pictures as in the example above are scaled up 2 times, horizontally, using a linear filter. In FIG. 3C, the up-scale factor is 3.
FIG. 3A illustrates an embodiment 300b of a resulting digital image, that includes noticeable distortion in high frequency content, as generated using only a linear FIR filter.
FIG. 3B illustrates an embodiment 300b of a resulting digital image, in which ringing/rippling is completely eliminated, as generated using only a linear FIR filter.
In FIG. 3B, it can be seen that that ringing/rippling is completely eliminated; however, in FIG. 3A, there is noticeable distortion in the high-frequency pattern.
FIG. 3C illustrates an embodiment 300c of a resulting digital image, in which ringing/rippling is reduced but that also includes softer/jagged edges, as generated using only a linear FIR filter. In FIG. 3C, there's no more ringing/rippling along the diagonal edges. However the edges are significantly more soft and jagged than before and the overall image quality is significantly reduced.
The prior art approaches to performing image processing simply do not provide an adequate solution that, among other things, preserves high-frequency detail (sharpness) in the output, prevent visible artifacts caused by aliasing, and suppress the rippling/ringing artifacts.